The present invention relates to an image encoder for encoding an image signal such as a digitized still-image signal or a differential image signal representing the difference between two frames in a moving image.
Conventional image encoders begin by performing a two-dimensional mathematical transformation such as a discrete cosine transform or a wavelet transform on the image signal. The transformed data are next rearranged in a one-dimensional sequence and quantized according to a certain quantizing rule. The quantized data are then coded by a suitable method such as an entropy coding method. As a result, the input image signal is usually compressed to a fraction of its original size.
One objective in image encoding is to achieve the maximum compression ratio. The key to attaining this objective is to have the mathematical transformation encode the image information in a relatively small number of non-zero values, or more precisely, in a relatively small number of values that will be quantized to non-zero values. Unfortunately, no single mathematical transformation works well for all images. Since the discrete cosine transform employs the cosine function, it succeeds excellently for image signals that vary in a periodic manner, but has less success with image signals in which sharp transitions (edges) appear at arbitrary locations. The wavelet transform is good at handling edges, but is less successful in dealing with periodic signals.
Conventional image encoders thus yield high compression ratios for some image signals and low compression ratios for other image signals. As a result, the average compression ratio falls far short of the desired maximum.